The present invention relates to magnetically confined plasma and ion sources for industrial processes such as plasma treatment, sputtering and plasma etching and to electric propulsion devices for space applications. Many closed drift ion sources have been proposed for these applications and several remain commercially viable. Publicly available articles by Kim and Zhurin, Kaufman and Robinson provide good general background information and other relevant references that pertain to magnetically confined plasma and ion sources. As described in these articles, prior art closed drift ion sources have both an inner and outer magnetic pole with a separate annular anode located between these poles. A closed drift magnetic field passes over the anode between these two grounded or electrically floating poles.
Extended Acceleration Channel Ion Sources
Past literature has divided closed drift ion sources into two classifications: Extended Acceleration Channel and Anode Layer. Although the demarcation is not consistent, the common dividing line is the ratio of channel width to channel depth. If the depth exceeds the width dimension, the ion source is classed as an extended acceleration channel type. In both this class and the anode layer class, an ion accelerating electric field is created in a racetrack shape by magnetic field lines roughly orthogonal to the electric field. Outside of this racetrack electrons move relatively freely without the presence of a magnetic field. As the electrons enter the ion source and attempt to reach the anode however, they are impeded by the crossing magnetic field lines. This causes the electrons to gyrate around and move along these magnetic field lines. An additional motion is a drift at right angles to both the magnetic and electric fields. This is termed the Hall current and is the purpose for the racetrack shape of the confinement region. In Madocks U.S. Pat. No. 7,259,378, assigned to a common assignee with the present invention, these motions are discussed in detail.
The Egorov U.S. Pat. No. 5,218,271 is typical of many extended acceleration channel sources in the prior art. Common to the prior art, this source has an annular anode with inner and outer high permeability magnetic poles. The Bugrova U.S. Pat. No. 6,456,011 B1 is of interest because this patent is directed to reducing the size of the ion source. The need for smaller, lighter ion sources is outlined. Bugrova reduces the source size by removing magnetic field generating components from the inner pole. The inner pole is still present but consists of only a high permeability material. The example given cites the outside diameter of the source to be 5 cm.
Anode Layer Ion Sources
Anode layer type ion sources are the second class of closed drift source. In anode layer sources, the closed channel depth is typically shorter or equal to the width. The closed drift published references discuss these sources. These sources have been commercialized for industrial uses. These sources were developed in Russia 40 years ago, and are largely considered public domain and few patents exist. However, U.S. Pat. Nos. 5,763,989 and 5,838,120 show typical configurations for an anode layer geometry. In the afore-mentioned U.S. Pat. No. 7,259,378, Madocks discloses an improved version of this source with pointed magnetic poles that focus the magnetic field in the magnetic gap. As can be seen in these and other anode layer ion sources, an annular anode is located between two, separate inner and outer magnetic poles.
End Hall Ion Sources
End hall ion sources are a variation of a closed drift ion source. In the end hall source, the inner magnet pole is lowered with respect to the outer pole to expose the sides of the annular anode. This is exemplified in both the Burkhart and Kaufman U.S. Pat. Nos. 3,735,591 and 4,862,032. With this geometry, a second electron confinement regime combines with a Penning style confinement of closed drift ion sources. The second confinement regime is mirror electron confinement in which electrons are partially confined along magnetic field lines by a gradient magnetic field. In the Burkhart and Kaufman patents and other prior art of this source type, e.g., Manley U.S. Pat. No. 5,855,745, the anode is again annular with the primary electron confining field lines passing from a central grounded or floating pole to an outer grounded or floating pole.
In the Sainty U.S. Pat. No. 6,734,434, a different end hall ion source configuration is presented. In Sainty the anode is not annular and there is no central floating pole. The anode fills the central area of the ion source and the magnetic field passes through the anode. Important to Sainty, the center of the anode is electrically conductive and is coated to insure the central top anode surface remains conductive. Electrons flowing from a filament reach the anode through the magnetic mirror at the center of the anode rather than by crossing magnetic field lines. This significantly lowers the impedance an electron experiences in trying to reach the anode and is different than the present invention.
Ion Sources with Sputter Magnetrons
The combination of sputter magnetron cathodes and closed drift ion sources is known in several configurations. In Morrison, Jr., U.S. Pat. No. 4,361,472, FIG. 13b shows a closed drift ion source connected as the anode to a sputter magnetron cathode. Morrison, Jr. teaches the use of separate power supplies to the cathode and anode (ion source) and the use of this tool in reactive sputtering. Scobey, U.S. Pat. No. 4,851,095, discloses another type of closed drift ion source using a sputter magnetron cathode to provide electrons to the ion source. In Scobey, separate power supplies are shown for the ion source and sputter magnetron cathode. In Manley, U.S. Pat. No. 5,855,745, an end Hall type ion source is used as the anode of a sputter magnetron cathode. Zhurin, U.S. Pat. No. 6,454,910, shows an Hall ion source with a sputter magnetron with separate power supplies for the ion source and sputter cathode.
Anodes for Sputter Magnetrons
Several prior art patents present apparatus for improved sputter magnetron anodes. In Meyer, U.S. Pat. No. 4,849,087, both inert and reactive gas is distributed in passageways though the anode. This is said to produce a stable plasma that uses the gases more efficiently. In this patent the anode is adjacent to the sputter magnetron and magnetic field lines are shown passing though the anode. Dickey, U.S. Pat. No. 5,106,474, teaches several anode configurations to maintain anode conductivity during magnetron sputtering of an insulating coating. FIGS. 8 through 11 show anodes with an array of magnets to guide electrons from the sputter cathode to the anode. Countrywood, U.S. Pat. No. 6,110,540, discloses a conductive anode that maintains conductivity by flowing inert gas through a pinhole and creating a plasma at the anode. In FIG. 7c of this patent the conductive anode is shown with plasma shaping magnets.